Dual mode smartcards typically contain an antenna embedded in a smartcard body and a module encapsulating a microprocessor coupled to a substrate surface, the latter provided with external leads for contacted smartcard operation to a card reader.
The smartcard body and module are matedly attached using a conductive adhesive. The conductive adhesive is typically deposited in each of two vias buried within the smartcard body. The vias are formed by milling the smartcard body to a depth sufficient to expose associated antenna leads buried in the smartcard body. After the vias (or antennas exposing cavities) are formed, they are filled with the conductive adhesive and the module properly aligned in mated fashion on top of the smartcard body, resulting in an electrical connection between the antenna and module by way of the conductive adhesive.
A critical element of smartcard manufacture is that dispensed volumes of conductive adhesive need to be precisely controlled in order not to overfill the vias. To date this has been inadequately achieved by using what is known as a non-conductive hot melt tape having a specific punch out pattern. The hot melt tape 10 includes three cut outs, an overmold receiving cavity (11) and two circular punch out holes 12, 13, as shown in FIG. 1. Circular punch out holes 12 and 13 are substantially the diameter of corresponding vias on the smartcards, over which the hot melt tape is superimposed during smartcard assembly.
Before combining the top surface of the smartcard body to the bottom surface of the module, the hot melt tape 10 is first adhesively bonded to the bottom surface of the module. The punch out holes 12 and 13 are dimensioned to expose corresponding electrical contact areas on the bottom surface of the module. Once the hot melt tape is affixed to the module, both vias on the smartcard body are filled with the conductive adhesive. The module (with hot melt tape) are then brought together with the smartcard body. In an ideal situation, when filling the vias, only enough adhesive is provided as necessary to fill the punch out holes 12, 13 sufficiently, and without overfill, so that contact is made between adhesive and the exposed electrical contact areas on the bottom surface of the module.
FIG. 2 shows the smartcard body facing surface of the hot melt tape with no overflow 14 having spread onto the surface area.
If overfill 14' does occur, and often times does occur, as shown in FIG. 3, it can lead to contamination of the hot melt tape around one or both vias and also result in insufficient adhesion of the module to the smartcard body. Insufficient adhesion around the via could result in a weak (or intermittent) electrical connection, particularly when the smartcard is bent--as expected to occur during normal use by its owner. Also, the overfill from adjacent connections could migrate towards each other and result in an electrical short, as shown by 14" in FIG. 4.
Controlling the dispensed volume of viscous materials like conductive adhesive has always been very difficult. Several issues, such as changing viscosity as the material cures, differing viscosity within the same tube of material, and differing viscosity between lots of material are the cause of the dispensing inaccuracies.
It would be a great advance in the art of smartcard manufacture to be able to provide a solution to the problem of overflowing conductive adhesive.